1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to the exhaust gas purifying apparatus having a filter (DPF: Diesel Particulate Filter) for trapping particulates (particulate matter) in exhaust gases of the internal combustion engine.
2. Description of the Related Art
A technique of providing a DPF which traps particulates in exhaust gases in the exhaust system of a diesel internal combustion engine and reducing an emission amount of particulates is conventionally and widely used. If a failure such as a crack or a hole occurs in the filter element of the DPF, the filtering capability of the DPF will be deteriorated and the emission amount of particulates will increase. Therefore, it is necessary to quickly detect such failure.
Japanese Patent Laid-open No. 2004-308454 shows a method of providing a pressure sensor on the downstream side of the DPF, calculating a difference between the maximum value and the minimum value, i.e., a pulsation amplitude, of the detected pressure during engine operation, and determining that a failure occurs when the calculated pulsation amplitude is outside of a predetermined range.
According to the technique shown in Japanese Patent Laid-open No. 2004-308454, a leak amount of particulates due to the failure cannot be detected. Therefore, it is difficult to satisfy the demand of detecting a failure before the leak amount of particulates exceeds a predetermined amount due to reduction in the particulate trapping capacity of the DPF. Further, according to the technique shown in Japanese Patent Laid-open No. 2004-308454, complicated calculations are necessary for monitoring and analyzing the pulsation amplitude of the exhaust pressure.
On the other hand, it is considered to be effective to provide a particulate sensor that directly detects leaked particulates on the downstream side of the DPF. However, development of such sensor has not yet been completed.